Suggestion: Allow mods to define custom conditions for trains and platforms.

Reason: Allows mods to create niche stops that aren't in the base game, like "no filtered cargo slot is empty"

Possible API:
Adding conditions:
script.add_train_stop(handler)
Where handler is a mod-defined function that ...

For items produced on planets, you could also set the item's weight so high that it cannot be sent into space, which would for at least some cases (esp. where the item can only be produced on one planet).